This invention relates to signalling systems, and more particularly to telephone systems in which dual tone multifrequency signals are used for signalling and must therefore be detected and identified.
Dual tone multifrequency ("DTMF") signals are often used for signalling in communications systems in which telephone transmission is involved because a great many telephone instruments can generate these signals, and because the frequencies involved are within the telephone transmission band. For example, in so-called voice processing or voice mail systems, the voice processor should be able not only to record voice messages for subsequent playback to the intended recipient, but should also be able to respond to various DTMF control signals originated by the person desiring to leave or retrieve a message and generated by that person using the buttons on his or her push-button telephone. Examples of the kinds of control signal information the person desiring to leave a message may want to transmit are the telephone extension number of the person for whom the message is intended, the last name of the person for whom the message is intended, or a code indicating the importance or priority of the message. Similarly, such DTMF control signals may be used by the person leaving or receiving messages to request various types of verbal prompts, instructions, menus, or helps, to skip or truncate unwanted verbal instructions, to cancel, skip, transfer, or broadcast messages, etc. Thus, the ability to quickly and correctly detect and identify DTMF signals is absolutely essential to modern voice processing systems.
It will also be appreciated that DTMF signals are often accompanied by many other types of non-DTMF signals (e.g., voice signals and/or varying levels of noise) which the voice processing system must not falsely recognize as DTMF signals. Voice signals, in particular, may sometimes have some of the characteristics of DTMF signals. It will be apparent from the above-mentioned many uses of DTMF signals in controlling voice processing systems that any false recognition of non-DTMF signals as DTMF signals will play havoc with the proper functioning of the voice processing system.
In view of the foregoing, it is an object of this invention to provide improved and simplified methods and apparatus for detecting and identifying DTMF signals.
It is a more particular object of this invention to increase the sensitivity and reliability of DTMF signal detection and identification methods and apparatus, and to do so with relatively simple algorithms which do not consume large amounts of computational resources.
It is still another object of this invention to provide DTMF detection and identification methods and apparatus which operate in noisy environments with no false detection or incorrect identification of DTMF signals as a result of the presence of non-DTMF signals such as noise or voice.